Accidents in which planes or helicopters fly into power lines happen once or twice a year in Norway, and almost weekly in the USA. These accidents often result in the loss of human life and substantial material damage. The accidents are a clear indication that today's marking of power lines and other aerial obstacles is not good enough. Marking of aerial obstacles using a system for avoidance of collision between an aircraft and an obstacle, hereinafter called OCAS, can reduce the number of accidents of this type considerably.
U.S. Pat. No. 5,351,032 discloses a short-range radio system, wherein the system provides both audio and visual warnings to the pilot of an aircraft of an upcoming power line. The system comprises a small narrow-band transmitter which utilises power from the power line, wherein the narrow-band transmitter may be installed on top of a power line tower or within already existing warning balls on the power line. The narrow-band transmitter provides a continuous warning signal to aircraft that approach the deployed narrowband transmitter, and a receiver installed on board the aircraft capable of receiving the warning signal from the narrow-band transmitter will, upon receipt of the warning signal, trigger both an audio and a visual alarm to alert the pilot of the aircraft to the potential hazard. Thus, the system requires that a part thereof, in this case a systemadapted receiver, should be located in the aircraft, and will only be effective in warning aircraft that have a part of the system installed therein. Therefore, aircraft that do not have a receiver of this kind installed will not be able to take advantage of the possible warning that the narrow-band transmitter could give. Furthermore, the system does not include any means for determining the distance between the aircraft and the obstacle, and it will thus be a matter of chance whether the receiver in the aircraft picks up the warning signal at a short or a long distance, which may result in a warning that is too late or in an unnecessary warning to aircraft which are at a great distance, but nevertheless within the maximum range that is determined by the propagation potential of the radio waves, the presence or absence of noise sources and the sensitivity of the receiver in question.
Thus, it is an object of the present invention to provide a solution that can give a warning to an operator of an aircraft in order to prevent collision between an aircraft and an obstacle, which overcomes the drawbacks of the previously known solutions.